Various abbreviations that appear in the specification and/or in the drawing figures are defined as follows:                MRC maximum ratio combining        SIR signal to interference ratio        HSDPA high speed downlink packet access        DPCH dedicated physical channel        DL downlink        QPSK quadrature phase shift keying        16-QAM 16-state quadrature amplitude modulation        CQI channel quality indicator        FDD frequency division duplex        TDD time division duplex        WCDMA wideband code division multiple access        
Exemplary Rake receiver embodiments can be found in commonly owned U.S. Pat. No. 6,215,814, Rake Receiver, Juha Ylitalo, Peter Muszynski, Esa Tiirola and Toni Neffling; U.S. Pat. No. 6,741,665 B2, Method and Apparatus Providing an Amplitude independent Automatic Frequency Control Circuit, Thomas J. Kenney and Jukka Tapaninen; and U.S. Pat. No. 7,245,652 B2, Rake Combiner for a CDMA Rake Receiver, Alice Wilson.
As is explained in U.S. Pat. No. 6,215,814, in a CDMA system a Rake receiver is used for separating multipath propagated signal components after reception. In general, the signal components are then separated from each other at least by using part of a spreading code (by a chip). The Rake receiver comprises Rake fingers where in each finger despreading and diversity combination take place. On the radio path the signal will also include, in addition to the desired signal, noise and interference caused by other users or systems. In systems utilizing diversity the influence of noise and interference can be decreased by using, for example, the MRC method.
In the conventional Rake receiver the combining of the multi-path signals can be accomplished by using different methods, in addition to the MRC method, such as by using equal gain combining and signal-to-interference ratio (SIR) combining. The end result is an indication of how multi-path signals are weighted before summing. Of these methods SIR combining is typically preferred since it gives the best performance. However, SIR combining is significantly more complex than other approaches, resulting in sub-optimal methods, such as MRC, being used for practical reasons.
One drawback to the use of MRC Rake is that under certain conditions it can result in performance that is less than what would be achieved where some of the multi-path signals are not combined at all (in the general case there should always be at least one received multi-path).
If a RAKE finger is allocated to a time instant where there in reality is no desired signal, the finger will add interference only to the combined signal, Therefore, in a typical MRC Rake implementation a so-called combining threshold is used to indicate the power value that a perceived or actual single multi-path signal has to exceed so that it will be combined in the MRC Rake receiver. This combining threshold value can be defined, for example, as a fixed offset from noise floor power (in general, a most typical case defines the combining threshold as a fixed value, rather than in relation to the strongest multi-path.)
In U.S. Pat. No. 7,283,578 B2, “System and Method to Adjust Searcher Threshold parameter of RAKE Receiver”, Che-Li L Kin and Sheng-Jie Chen, an adjustable threshold is used in a searcher unit 122 of a Rake receiver.